An electrolyte membrane-electrode assembly used for a polymer electrolyte fuel cell (PEFC) is obtained by bonding a first gas diffusion electrode as an anode and a second gas diffusion electrode as a cathode to a film-shaped hydrogen ion-conductive polymer electrolyte membrane as an electrolyte. The gas diffusion electrode is composed of a gas diffusion layer and a catalyst layer, and the gas diffusion layer is constituted of porous carbon paper or the like. The catalyst layers of the anode and cathode are constituted of noble metal fine particles and carbon particles carrying these fine particles thereon.
As shown in FIG. 10(b), an electrolyte membrane-electrode assembly for the PEFC is obtained by bonding gas diffusion electrodes 146 and 147 to a film-shaped polymer electrolyte membrane 141 as the electrolyte. The polymer electrolyte membrane 141 is typically supplied with a roll.
FIG. 10(a) shows one example of production methods of the electrolyte membrane-electrode assembly, and herein, carbon paper (gas diffusion layers) 142 and 144 with catalyst layers 143 and 145 formed thereon are press-contacted with the polymer electrolyte membrane 141 by hot pressing. Another method is to form the catalyst layer on the polymer electrolyte membrane in advance by transfer-printing, printing or the like, and carbon paper is then press-contacted therewith.
In the catalyst layer 143 on the anode side, a reaction represented by the formula (1) occurs:H2→2H++2e−  (1),while, in the catalyst layer 145 on the cathode side, a reaction represented by the formula (2) occurs:½O2+2H++2e−→H2O  (2)When the above reactions occur, protons (hydrogen ions) generated in the anode migrate to the cathode through the polymer electrolyte membrane 141.
Such a PEFC is required to generate a high output voltage, for which it is of necessity for a polymer electrolyte membrane used to have high proton conductivity, namely, to have low internal resistance. In order to obtain high proton conductivity, there are needs for the use of a material with high proton conductivity for the polymer electrolyte membrane and for the use of as thin a membrane as possible.
For a polymer electrolyte membrane in a typical PEFC used has been a polymer electrolyte membrane made of perfluorocarbon sulfonic acid ionomer, represented by Nafion 112 produced by Du Pont in the US, and a membrane having a thickness of about 30 to 50 μm has been put to practical use.
A polymer electrolyte membrane made of perfluorocarbon sulfonic acid ionomer which has higher proton conductivity than aforesaid Nafion may by exemplified by a Flemion S H membrane produced by Asahi Glass Co., Ltd.; however, there is a problem with this membrane that the membrane is more fragile and breakable than Nafion 112 because of containment of the sulfonic acid group therein. A membrane in practical use therefore has a thickness of not less than about 50 μm.
In order to make the polymer electrolyte membrane thinner, for example, Japanese Laid-Open Patent Publication No. Hei 08-162132 discloses a method in which a porous cloth made of polytetrafluoroethylene is used as a core member and in the porous part thereof, a polymer electrolyte resin is impregnated so as to form a polymer electrolyte membrane imparted with high intensity.
The examples of specific products produced by this method may include a GORE-SELECT membrane produced by JAPAN GORE-TEX INC. This type of membrane with a thickness of as thin as about 20 to 30 μm has been in practical use by using a reinforcing material.
Next, as for the production method of the electrolyte membrane-electrode assembly, there is a method in which an ink-like or paste-like mixture of the catalyst and the electrolyte, containing the catalyst, is applied onto the surface of the electrolyte membrane or of the gas diffusion layer by printing, spraying or the like. In either method, after application of the mixture, the electrolyte membrane and the gas diffusion electrode are bonded to each other by hot pressing or the like (e.g., Japanese Laid-Open Patent Publication No. Hei 6-203849, Japanese Laid-Open Patent Publication No. Hei 8-88011 and Japanese Laid-Open Patent Publication No. Hei 8-106915.)
There is another production method of the electrolyte membrane-electrode assembly, in which the catalyst layer formed on the base material in advance is transfer-printed to the electrolyte membrane by hot pressing or thermoroll (e.g., Japanese Laid-Open Patent Publication No. Hei 10-64574). This method is excellent from the viewpoints of control on and uniformity of the film thickness of the catalyst layer, production efficiency and cell performance.
However, the conventional production method comprises a step of handling the electrolyte membrane without using the base material for the electrolyte membrane. Thereby, when the electrolyte membrane with a film thickness less than 20 μm and lower intensity is used, for example, it has been extremely difficult to produce the electrolyte membrane-electrode assembly without breakage of the electrolyte membrane.
That is to say, when drawing stress or sheering stress is directly applied to the electrolyte membrane with a thin film thickness in the state that the base material for the electrolyte membrane is not present in the process of producing the electrolyte membrane-electrode assembly, defects such as a pinhole, breakage and crack easily occur. These defects cause occurrence of crossover of a fuel gas and air or of short-circuit in the electrolyte membrane-electrode assembly, raising a problem of significant deterioration of performance of the PEFC.
Moreover, since the polymer electrolyte having high proton conductivity such as perfluorocarbon sulfonic acid ionomer contains a hydrophilic group such as the sulfonic acid group in the molecular chain, ionomer soluble to water tends to gradually flow into the gas diffusion layer such as carbon paper in operation of the fuel cell. For this reason, there has been a problem that a reaction area of a triphasic interface, formed of a pore as a supply channel of a reaction gas, a polymer electrolyte having proton conductivity due to containment of water and an electrode material as an electron conductor, gradually becomes smaller, decreasing cell output. Further, there has been another problem that, when a current collector having a gas flow channel arranged in the outside of the assembly of the polymer electrolyte membrane and the electrode is made of metal, the assembly is gradually corroded with dissolved acidic ionomer, significantly lowering reliability of the fuel cell.
In order to solve the above problems, therefore, it is an object of the present invention to provide an electrolyte membrane-electrode assembly for a polymer electrolyte fuel cell with low internal resistance and of great power, which can use perfluorocarbon sulfonic acid ionomer having high proton conductivity and comprises a thin polymer electrolyte membrane capable of being formed on a catalyst layer.
It is also an object of the present invention to provide an electrolyte membrane-electrode assembly for a polymer electrolyte fuel cell, which has a uniform film thickness to cause no clogging in the porous part of the catalyst layer of the gas diffusion electrode by preventing soakage of the raw material solution of the polymer electrolyte membrane into the porous part, thereby having excellent electrode properties.
It is further an object of the present invention to provide an electrolyte membrane-electrode assembly using a polymer electrolyte having high proton conductivity and exhibiting excellent durability and high performance, and a polymer electrolyte fuel cell constituted using this assembly.